The invention relates to a semiconductor diode laser--often called diode laser for short hereinafter--surrounded by a medium having a refractive index approximately equal to one, with a semiconductor body comprising a semiconductor layer structure situated on a semiconductor substrate, which structure comprises at least one active layer situated between two cladding layers and a pn junction which, given a sufficient current strength in the forward direction, is capable of generating coherent electromagnetic radiation in a strip-shaped active region situated within a resonant cavity and forming part of the active layer, which resonant cavity is bounded by end faces which are substantially perpendicular to the active region and of which at least one is provided with a covering layer. The invention relates in particular to a diode laser which emits in the sub-micron part of the spectrum. Such a diode laser is particularly suitable for use in the read and/or write head of information-processing systems such as laser printers, bar code readers, and read and/or write devices for optical registration such as CD(Audio) and CDROM (Data) discs. The invention also relates to a method of manufacturing such a laser.
Such a diode laser is known from the publication "Reliable high-power (40 mW) operation of transverse-mode stabilised InGaAlP laser diodes with strained active layer" by K. Nitta et al., published in Electron. Lett. vol. 28, no. 11, 21th May 1992, pp. 1069-1070. The diode laser described therein emits radiation with a wavelength of 698 nm from an active region which forms part of an active layer situated between an n-type cladding layer and a p-type cladding layer. The active region lies within a resonant cavity which is bounded in longitudinal direction by two end faces which are practically perpendicular to the active region and of which one is provided with a covering layer of Al.sub.2 O.sub.3. Such a layer passivates the end face which acts as an exit face, while the thickness of the covering layer is chosen such that the layer has a desired reflection, i.e. a comparatively low reflection of 10%. The other end face bounding the resonant cavity is provided with a covering layer of high reflection and comprises a dielectricum-Semiconductor, i.c. an Al.sub.2 O.sub.3 --Si multilayer. The medium in which the diode laser is present often comprises a (inert) gas atmosphere, but it may alternatively be vacuum. As the publication shows, the known diode laser combines a high emitted optical power with a long life.
A disadvantage of the known diode laser that it is not satisfactorily in practice in certain cases because an even higher emitted optical power and an even longer life are desired. This is especially the case in the application mentioned above as a write head in an optical registration system.